1. Field of the Invention
The invention relates generally to the field of pipe centralizers used to position pipes within wellbores drilled into the Earth. More specifically, the invention relates to structures for pipe centralizers that have increased restoring force, without corresponding increase in running force. The invention also relates to structures for centralizers that are self-sealing to enable moving through a wellbore pipe string sealing device.
2. Background Art
Centralizers are used to laterally or radially position a pipe or pipe “string” within a wellbore drilled into the Earth. A common type of centralizer is the so-called “bow spring” centralizer. A bow spring centralizer includes a plurality of circumferentially spaced apart single-leaf springs coupled to one or more devices arranged to affix the springs to the exterior of the pipe to be positioned in the wellbore. The leaf springs provide a force known as “restoring force” to laterally urge the pipe away from the wall of the wellbore. At the same time, the bow springs are laterally compressible so that the pipe may be moved along the interior of the wellbore notwithstanding the presence in the wellbore of small diameter restrictions and other obstacles to longitudinal movement of the pipe along the wellbore.
Examples of bow spring centralizers are described in U.S. Pat. No. 7,159,668 issued to Herrera and U.S. Pat. No. 6,457,519 issued to Buytaert. The centralizer described in the '519 patent is intended to address a particular problem associated with bow spring centralizers, namely how to minimize the “staring force”, which is the force required to insert the centralizer into interior of the wellbore and the “running force”, which is the amount of force required to move the pipe longitudinally along the wellbore with such centralizers affixed to its exterior, while maximizing the restoring force. Specifications for the amount of restoring force, and proper use of centralizers are described in a document entitled, Specifications for Bow-Spring Centralizers, API Specification 10D, fifth edition, American Petroleum Institute, Washington, D.C. (1994). Generally speaking, casing centralizers are made to center a particular outside diameter (OD) pipe within a particular nominal diameter wellbore or outer pipe. The pipe OD is selected by the wellbore operator to closely match, for example, the wellbore diameter, which primarily related to the diameter of the drill bit used to drill a particular segment of the wellbore.
More recently, techniques have become known in the art to drill wellbores while maintaining a selected fluid pressure in an annular space between the wellbore wall and the pipe used to drill the wellbore. See, for example, U.S. Pat. No. 6,904,981 issued to van Riet and U.S. Pat. No. 6,352,129 issued to Best. Drilling techniques such as those disclosed in the foregoing patents typically require the use of a “rotating control head” at the upper end of the wellbore in order to control the pressure in the body of fluid in the annular space. A rotating control head is a device which closes the annular space while simultaneously enabling longitudinal and rotational movement of the pipe therethrough. Using a rotating control head with centralizers affixed to the exterior of the pipe may present particular difficulties in providing sufficient restoring force while maintaining the ability to sealingly move the pipe through the rotating control head.